The wide-spread availability of hybrid fiber/coaxial (“HFC”) networks, along with the broadband nature of the HFC networks, are allowing new and improved services to be brought into the home and/or office of consumers. Services that have traditionally been provided via other mediums, such as local telephone service and Internet access, are now available through HFC networks that are owned, operated, and/or utilized by cable operators.
The provision of local telephone service through HFC networks is rapidly expanding. Large multiple systems operators in the United States are achieving penetration rates of 10 to 20 percent of homes passed with more than 95 percent of acquired subscribers dropping their old carrier in favor of the cable operator. Today, more than 200,000 lines of telephone service are provided by means of HFC around the world, with more than 40,000 of these lines being in the United States.
Local telephone service provided through an HFC network has several advantages over the traditional analog loop provided via copper wiring. One such advantage is that unlike the analog loop, HFC provisioned telephone service is not as susceptible to cross-talk, electrical noise, inductance capacitance, lightning and power cross. In addition, the voice quality provided by use of an HFC network is improved over that of a copper analog loop by improving the effect of jitter, attenuation distortion and other key voice quality parameters.
Traditional telephone service over the PSTN and local-loops has achieved a high-level of reliability and availability. The provisioning of traditional services over an HFC network has not been held to the high standards that have come to be expected for the provisioning of telephone service. However, to be a competitive and viable service, the cable operators have had to identify technical problems undermining the reliability and availability of telephone service through an HFC network and then resolve these technical problems.
One such technical problem that has not been fully addressed and resolved to date is maintaining the synchronization of transmitting devices within an HFC network, especially when network errors occur. In an HFC telephone network, a network interface unit (“NIU”), also referred to as VOICEPORT or a CPE, interfaces between subscriber equipment and the rest of the HFC network. A host digital terminal (“HDT”) interfaces the HFC network to the public switched telephone network (“PSTN”). The HDT also operates as a controller for each of the NIU's. In operation, the NIU's transmit in a time-division-multiple-access format under the control of the HDT. For instance, the HDT assigns time-slots to each of the NIU's. When the topology of the HFC network changes, such as when a fiber cable is severed and a redundant path is switched in, the propagation delays associated with the transmissions of one or more NIU's may be altered. This results in the aforementioned loss of synchronization for the transmitting devices on an HFC network.
Using existing technology, a long, cumbersome process is required to overcome a loss of synchronization of the transmitting devices on an HFC network. This process can take on the order of 30 to 45 minutes, if it can be resolved at all. This amount of time is not acceptable to typical consumers because it is way below the standards offered by traditional telephony networks. Therefore, there is a need in the art for a system and method to detect and alleviate transmission synchronization problems within an HFC network and thereby reduce service outages following an HFC route switch.
One technique that has been used to recover transmitters after a route switch is to use a TDMA search algorithm. The TDMA search algorithm measures propagation delays of transmitters by examining the time delay of their responses when prompted to transmit a pulse at the start of a TDMA frame. In this type of system, the transmitted pulses fall within the header portion of the TDMA frame. However, for larger networks, this technique fails. One cause of this failure is that the propagation delay may be so large that the transmitted pulse will fall within the channel space of the TDMA frame. The large propagation delays result in corrupting the data and making it difficult for the pulse to be detected. In addition, as the size of the network increases, the TDMA search algorithm is too slow. Therefore, it is clear that there is a need in the art for a system and a method of timely recovering transmitters within a large network.